1. Field of the Invention
The present invention relates to a straight travelling apparatus for heavy construction equipment, and more particularly to such an apparatus capable of continuously maintaining the straight travelling function of construction equipment such as excavators or cranes even when a working unit of the construction equipment is operated during movement of the construction equipment.
2. Description of the Prior Art
Generally, travelling construction equipment such as excavators and cranes are constructed to operate actuators (travel motors) of its travelling system and actuators (boom cylinder, arm cylinder, bucket cylinder and the like) of its working system use a flow of fluid delivered from a single pump.
Such a construction is illustrated in FIG. 1. As shown in the drawing, a first hydraulic pump P1 and a second hydraulic pump P2 both having the same displacement are provided to be driven by a single engine. The first hydraulic pump P1 serves to supply operating fluid for the right travel motor TR and actuators a, b, c and d of a working system whereas the second hydraulic pump P2 serves to supply operating fluid for a left travel motor TL and other actuators a', b' and c' of the working system.
Although the reference symbols TR and TL in FIG. 1 have been shown as respectively denoting the right travel motor-side control valve and the left travel motor-side control valve, they will be described as respectively corresponding to the right travel motor and the left travel motor in the following description for convenience. For the same purpose, the reference symbols a, b, c, d, a', b' and c' will be described as respectively corresponding to the actuators of working system in the following description, even though they have been shown as respectively denoting control valves of the actuators of the working system.
As the right and left travel motors TR and TL are driven after an activation of the engine in the conventional construction equipment having the above-mentioned construction, a flow of fluid delivered from the first hydraulic pump P1 is fed to the right travel motor TR via a fluid line connected between the first hydraulic pump P1 and the right travel motor TR while a flow of fluid delivered from the second hydraulic pump P2 is fed to the left travel motor TL via a fluid line connected between the second hydraulic pump P2 and the left travel motor TL. Accordingly, the construction equipment can travel straight because the first and second hydraulic pump P1 and P2 have the same displacement.
When at least one of the actuators of the working system is actuated under the condition that the construction equipment travels straight, for example, when the arm cylinder c' is supplied with the operating fluid to actuate an arm, the fluid flow delivered from the second hydraulic pump P2 is distributively fed to both the left travel motor TL and the arm cylinder c' via parallel fluid lines. As a result, the amount of fluid supplied to the left travel motor TL becomes less than the amount of fluid supplied to the right travel motor TR. This results in a failure to keep the the construction equipment travelling straight and thereby creating a left declination of the construction equipment.
Therefore, it is absolutely needed to provide a straight travelling device for avoiding unexpected declination of the construction equipment and keeping of the construction equipment travelling straight even when a working unit is operated during movement of the construction equipment.
Referring to FIG. 2, there is illustrated a conventional straight travelling device designed to achieve supply of operating fluid in a fashion that the first and second hydraulic pumps serve to independently supply fluid flows therefrom to the left and right travel motors when there aren't any working units being operated during movement of the construction equipment whereas when at least one working unit is being operated, one of them serves to supply its fluid flow to both the left and right travel motors while the other serves to supply its fluid flow to the working unit.
This will be described in more detail in conjunction with FIG. 2. Pilot oil delivered from a pilot pump P is distributively fed to a first pilot fluid line Pi1 and a second pilot fluid line Pi2. The first pilot fluid line Pi1 is designed to be connected to the main fluid line via inner fluid passages of selector valves s1 and s2 directly connected to respective control valves of the right and left travel motors TR and TL so that its pilot oil can drain into the main fluid line. On the other hand, the second pilot fluid line Pi2 is designed to be connected to the main fluid line via inner fluid passages of selector valves s3, s4, s5, s6, s7, s8 and s9 directly connected to respective control valves of all actuators a, b, c, d, a', b' and c' or the working system so that its pilot oil can drain into the main fluid line.
Each of the inner fluid passages of selector valves s3, s4, s5, s6, s7, s8 and s9 is designed in a fashion that it is opened in a neutral state of each associated actuator directly connected thereto and shut off during operation of the actuator. When either of the two actuators TR or TL of the travelling system operate, accordingly, the first pilot fluid line Pi1 increases in pressure. When any of the actuators a, b, c, d, a', b' and c' of the working system operates, the second pilot fluid line Pi2 increases in pressure.
When the first pilot fluid line Pi1 increases in pressure it is shut off at its drain side due to the operation of either of two actuator TR or TL of the travelling system.
The increased pressure of the first pilot fluid line Pi1 serves to move a spool of the flow switching valve 201 connected between the first pilot fluid line Pi1 and its drain side. By this movement, the second pilot fluid line Pi2 is communicated with a fluid line 202 which is a pilot fluid line for moving a spool of a valve 203 for straight travel. If any of the actuators a, b, c, d, a', b' and c' of the working system do not operate under the above-mentioned condition, the spool of the valve 203 for straight travel can not move against resilience of a valve spring 205 provided at the valve 203 because the pressure in the second pilot fluid line Pi2 is zero. In this case, accordingly, the straight travel valve 203 is maintained at its I-state. On the other hand, an operation of at least one of the actuators a, b, c, d, a', b' and c' of the working system results in shutting-off the second pilot fluid line Pi2 at its drain side and thereby increasing the pressure of the second pilot fluid line Pi2. The Increased pressure of the second pilot fluid line Pi2 is applied to the spool of the straight travel valve 203 via the fluid line 202 communicated with the second pilot fluid line Pi2, thereby causing the spool of the valve 203 to move against the resilience of the valve spring 205. As a result, the straight travel valve 203 is switched to its II-state.
However, if neither of the actuators TR nor TL of the travelling system operates under the condition that at least one of the actuators a. b, c, d, a', b' and c' of the working system is operating, then the straight travel valve 203 is still maintained at its I-state. This is because the fluid switching valve 201 is maintained at its shut-off state preventing communication between the second pilot fluid line Pi2 and the fluid line 202 when neither of the actuators TR nor TL of the travelling system is operating, thereby disabling the spool of straight travel valve 203 to move even though the operation of at least one actuator of the working system provides a sufficient pressure in the second pilot fluid line Pi2.
Under the condition that either actuator TR or TL of the travelling system and at least one of the actuators, a, b, c, d, a', b' or c' of the working system is operating in the II-state of the straight travel valve 203 in the above-mentioned conventional construction, accordingly, the fluid flow delivered from the first fluid pump P1 is supplied to both the right and left travel motors TR and TL of the travelling system while the fluid flow delivered from the second fluid pump P2 is supplied to the actuators a, b, c, d, a', b' and c' of the working system. Thus, the right and left travel motor TR and TL are always supplied with the same fluid amount irrespective of whether or not the working system is operating, thereby achieving the straight travelling function.
However, the conventional straight travelling device involves a problem that the construction of fluid lines and hydraulic elements added to achieve the straight travelling function is very complex. This results in an increased number of manufacturing and assembling steps and thereby difficulties in maintenance and repair. Moveover, the conventional device requires a large number of constituting elements and thereby an expensive manufacturing cost.